1. Field of the Present Invention
The present invention pertains to a video signal mixer circuit which can be more especially applied to the obtaining of high-resolution images with television cameras having solid-state circuit matrix sensors known as CCDs (Charge Coupled Devices).
2. Description of the Prior Art
CCD sensors incorporate, on one and the same silicon surface, a matrix of sensitive dots and the device for the serial reading of all these sensitive dots used to form a video signal. There are two main types of architecture: frame transfer devices and line transfer devices.
By nature and construction, solid-state sensors have many advantages as compared with silicon tubes. These advantages relate chiefly to their compactness and to the small voltages used. They are also noteworthy for their high resistance to shocks and vibration, the possibility of being cooled in a simplifed way, their longer lifetimes, easy maintenance, etc.
Prior art methods for heightening the resolution of CCD cameras include fitting them with two interline transfer type sensors having a great number of cells per line (up to 500 dots per line). These cells are not joined to each other. The sensitive zone and the memory zone are interlaced with each other and not on top of each other as in the so-called frame transfer type of organization.
The memory zone of these matrices is organized in a series of vertical registers interposed between the columns of sensitive cells. These vertical registers constitute blind zones of a width comparable with that of the columns of sensitive dots. It is therefore possible to associate two sensors of this type optically in displacing them by half a pitch along the line horizontally and by superimposing them vertically in order to double the horizontal resolution.
The optical assembly is done on a separating prism placed between the lens and the CCD sensors. The prism has a flat semi-transparent surface which divides the beam into two so that the sensor "sees" the same image. The two CCD sensors are positioned precisely with respect to each other, and are offset by half a pitch as stated above.
For the reading of the sensors, it is possible to use two clock signals at the dot frequency, one of these signals being shifted by half a cycle with respect to the other to correspond to the spatial offset between the matrices along the horizontal lines. After processing by alignment with the black level and automatic gain control, the video signals of the two sensors can be multiplexed at the dot rate to select the alternating dots of each sensor.
This simple technique has many disadvantages, resulting essentially from the non-homogeneity of the two sensors. For, during the manufacturing process, the doping of silicon is not homogeneous both inside the strip and on each slice cut out of the strip where the said doping varies from the center to the edge and from one slice to another. Consequently, the diodes formed on the matrix have differing detection sensitivities and small variations between one another. The two sensors therefore have different detecting levels for the same lighting conditions. Furthermore, changes according to temperature result in a black-reducing leakage current which will also differ. The architectural aspect has little effect on the variations except when the components involved come from different manufacturers.
It is therefore, necessary to compensate for the differences between the sensors, but differences in sensitivity cannot easily be compensated for by electronic techniques. To obtain a good-quality image, the level of the video signal given by the two sensors would have to be servo-controlled with a precision of about 1%, and there would have to be identical levels of black when the temperature varies, and it would be necessary to select sensors with sensitivities which are substantially identical and homogeneous on their surface.
There is no immediate method to cope with all these constraints when using the equipment, and they have not been coped with at the level of the manufacturer.
An object of the invention is to propose a video mixer which will remove these disadvantages by using standard sensors. It is known, especially with contour-correction techniques or with high-definition television images, that the video data can be separated into two parts, a first part corresponding to a low frequency spectrum between 0 and about 500 Khz (this latter value is given as an indication and it is possible to choose a limit which is quite far from this value) and a high-frequency spectrum located above the low-frequency limit selected. The low-frequency information is the part sensitive to the flaw represented by non-homogeneity. It corresponds, in the image, to broad zones of dark or bright colors. This low-frequency information does not relate to the details and fineness, and therefore excludes sudden variations, especially the contours. With this spectrum, the smallest dot is represented by a zone corresponding to about 3% to 4% of the width of the screen. The high-frequency information, on the contrary, corresponds to the sharpness of the image at the straight edges and, therefore, to the contours and to sudden variations. This high-frequency value is limited to about 7 to 8 MgHz for presently used sensors.